1. Field of the Invention
The present invention is related to a display system in which control data is communicated between a computer system and a display device.
2. Prior Art
The control data includes parameters for specifying the geometry and resolution of an image presented on the display device. In a display system comprising a raster-scanned display device such as a Cathode Ray Tuhre (CRT) display device, these parameters are determined by the rates and amplitudes of horizontal and vertical scan signals generated for producing the raster scan by electrical circuits in the display device. In order to generate the image, the scan signals are synchronised to video signals from a video source such as a computer system by synchronisation (sync) pulses also generated by the video source.
Some display devices can only operate in a single display mode in accordance with a single set of parameters. Other display devices can be configured to operate in any one of a number of display modes characterised by different sets of parameters. The latter will hereinafter be referred to as multiple mode display devices. In a display device controlled by a computer system it is desirable for the computer system to identify the type of the display device so that appropriate video and sync signals can be generated. Many examples of such computer systems, including the IBM PS 2 range, comprise a video graphics adapter (VGA) having an output port for connecting video and sync signals to a display device. The VGA also has logic responsive to the manner in which identification pins in the output port are terminated when connected to the display device. The logic identifies the type of display device connected to the VGA adapter from these terminations.
UK Patent No. 2,162,026 describes an example of a display system employing a multiple-mode display device receiving video and sync signals from a computer system display adapter. The display device can operate in any one of four display modes. The computer system can be instructed to provide sync pulses of either positive or negative polarities. Each polarity combination indicates a different display mode. The display device includes decoding logic for configuring the display device to operate in a particular display mode in response to predetermined sync pulse polarities.
The display systems of the prior art have the disadvantage that the display interfaces of the prior art can identify, and therefore generate appropriate control signals for, only a limited number of different display devices. This limitation arises because the number of pins available for display device identification and control is limited by the physical form of the output port.